1. Field of the Invention
The present invention relates to isocyanate-terminated quasi-prepolymers which are compatible with non-reactive blowing agents and to polyisocyanurate foams prepared therefrom. The quasi-prepolymers of the subject invention are prepared by reacting a stoichiometric excess of one or more organic isocyanates with a polyester polyol derived from the transesterification of dimethyl terephthalate process residue. The polyisocyanurate foams prepared from these quasi-prepolymers have exceptionally low friability.
2. Description of the Related Art
Rigid foams derived from polyisocyanate addition polymerization continue to be an important and expanding segment of the polyurethane industry. These rigid foams may be prepared by numerous methods resulting in foams having a wide variety of physical and chemical properties. One of the most important areas of application of rigid foam produced by polyisocyanate addition polymerization is in the area of foamed-in-place insulation. Such insulation is commonly found in refrigerator and freezer cabinets, chest-type coolers, refrigerated tractor-pulled trailers, and building panels and insulated doors. Increasing amounts are being utilized in buses, trains, and, particularly, in aircraft construction where the high insulating capacity is combined with the suitable physical properties to produce a variety of structural components.
Because of the types of applications which rigid foams lend themselves to, the flame or fire resistance of these foams has become a well recognized concern. While it is obviously impossible to render any organic material completely flame or fire-proof, much research has been done in order to increase the fire resistance of such foams.
A moderate degree of success has been accomplished by the addition of halogen-containing flame retardants, for example. These flame retardants are available commercially in both reactive and non-reactive forms. The reactive forms, often in the form of a halogenated polyether, polyester or polyester-ether polyol, are incorporated chemically into the foam, and are essentially non-volatile. The non-reactive flame retardants, on the other hand, are physically incorporated and generally are effective to impede initial flammability only, being volatilized at higher temperatures. Despite the incorporation of significant amounts of such flame-retardants, rigid foams are often too flammable for many uses for which they would otherwise be suitable. Further increasing the amount of flame retardant has proven to be unsuccessful, as the resulting foams have poor physical properties, particularly with regard to friability.
Another method of altering the flammability characteristics of rigid foams produced by polyisocyanate addition polymerization is to change the polymer linkages produced in the polymerization process. For example, foams containing predominately urethane linkages--polyurethane foams--are generally the most flammable. These foams are generally prepared by reacting nearly stoichiometric equivalents of hydroxyl-functional polyol and isocyanate in the presence of a urethane promoting catalyst, a blowing agent, and surfactant. The polyols utilized to produce rigid polyurethane foams generally have a functionality of three or more.
Foams containing significant quantities of urea linkages can be prepared by the substitution of polyamines in place of the conventional, urethane forming hydroxyl-functional polyols. Unfortunately, these polyamines, due to the substantially higher reactivity of the amino group compared to the hydroxyl group results in very rapid polymerization which is difficult to control.
Foams containing carbodiimide linkages may be produced by using catalysts which promote carbodiimide formation, or by using carbodiimide modified isocyanates. However these methods, while producing more flame resistant foams having superior physical properties, have not been utilized commercially to any great extent, partially due to the high cost of the starting materials, for example the carbodiimide modified isocyanates.
Perhaps the most successful of the fire-resistant rigid foams produced by polyisocyanate addition polymerization are the polyisocyanurate foams. These foams are made by trimerizing an organic isocyanate in the presence of a blowing agent and a suitable trimerization catalyst. Because these foams may be made without any polyol component, the resulting foam is generally superior to other polyisocyanate addition polymer foams in flame retardancy. The flame retardancy of the polyisocyanurate foams may be increased further by the addition of traditional flame retardants. When reactive flame retardants are added, either a polyisocyanurate-urethane or polyisocyanurate-urea foam is produced depending on the functional group type of the reactive flame retardant.
An improvement in the flame retardancy of all polyisocyanate addition polymer foams can be achieved if a "char former" is present. A char former is an ingredient which assists in the formation of an intumescent char on the outside of burning foam. The char not only slows flame spread, but it also serves as an insulator, shielding the inside of the foam from the heat of the fire.
In U.S. Pat. No. 4,237,239 is disclosed the preparation of polyisocyanurate foams wherein small amount of a crude polyester polyol derived from the process residue from dimethylterephthalate (DMT) bottoms is reacted with an organic isocyanate in the presence of a blowing agent and a trimerization catalyst. While the resulting polyisocyanurate-urethane foams contain minor amounts of urethane linkages which would ordinarily increase combustibility, the foams readily form an intumescent char which significantly decreases both the flammability and the amount of smoke evolved while burning.
However the process disclosed by U.S. Pat. No. 4,237,239 is difficult to implement as it requires as many as five different feeds into the mixing head due at least in part to the incompatibility of the very polar DMT derived polyester and the halocarbon blowing agent. Furthermore, the polyisocyanurate foams produced are quite friable, with weight losses, as measured by ASTM test method C-421. generally in excess of 24 percent, despite appreciable urethane-group content.
U.S. Pat. No. 4,425,444 discloses a process for the preparation of urethane-modified polyisocyanurate foams in which special surface active, amine-initiated polyoxyalkylene polyether polyols are added to the dimethylterephthalate derived polyester polyols to achieve compatibility with freon. The polyol mixture thus prepared is stable to separation for periods of greater than seven days. However the large proportion of urethane groups present in these urethane-modified polyisocyanurate foams, while significantly decreasing friability, cause the foams to be much less flame retardant than polyisocyanurate foams having a higher proportion of isocyanurate linkages.